The phenotypic modulation of vascular smooth muscle cells (VSMCs) from a quiescent, differentiated state to that of proliferating, "undifferentiated" state is a common pathogenic feature of vascular disease. We had previously shown that proliferating VSMCs express a mRNA with strong homology to the mouse ID (Inhibitor of Differentiation) gene and this expression was downregulated when these cells differentiated. In skeletal muscle, the product of the ID gene is thought to acta as a transdominant suppressor of the muscle regulatory/transcription factors that belong to the helix-loop-helix family of proteins. Expression of ID in proliferating skeletal muscle precursor cells and its downregulation preceding differentiation prevents HLH muscle transcription factors from activating skeletal muscle-specific gene expression in the proliferating precursor cells. Using a variety of molecular cloning approaches, we have identified and cloned the cDNAs for three putative ID-like proteins that are expressed in proliferating VSMCs. We have identified one of these as rat ID, the rate homologue of the original mouse ID cDNA. Rat ID is expressed at low levels in the vessels of mature rats, but at very high levels in proliferating VSMCs derived from these vessels. Its expression is significantly reduced by conditions that bring about growth arrest of the cells, such as confluence, serum withdrawal, or contact with basement membrane matrices. VSMC cultures established from the vessels of older rats also express high levels of rat ID as they proliferate, but unlike "younger" VSMCs do not downregulate ID levels in response to those manipulations that lead to growth arrest in the "younger" cells.